Subnet.SetSpace should be available

[John A Meinel]

Right now you can declare what space a subnet is in when you AddSpace, and then you can remove it with:
  subnet.EnsureDead()
  subnet.Remove()

However, we should support moving subnets between spaces. We need it in the test suite, but we'll also want to have it for future support as we auto-detect subnets from a provider and need to move them into declared spaces.

[Heather Lanigan (hmlanigan)]

Move a list of CIDRs from their space to a new space:
 juju move-to-space db-space 172.31.1.0/28 172.31.16.0/20